Electrical circuitry has always been vulnerable to damage caused by power surges due to electrostatic discharges (ESD) and/or electromagnetic pulses (EMP) or electromagnetic interference (EMI). Therefore terminators have been developed which serve to either filter these high energy conditions or to burn out and not allow any voltage to pass when surge conditions occur.
These adapters have been modified and updated as the state of the art of electrical circuits has changed. In the telecommunications field, with the advent of the new ISDN standards, it is again necessary to develop new equipment.
The ISDN terminator of the present invention is a small but, necessary part of the overall network. The terminator provides a 100 ohm bus termination function for each tap on the network. This termination allows each tap to be tested by the system, whether or not a customer is connected. If this termination is not seen by the system, trouble is suspected and the networks integrity could be compromised.
Conventional analog telecommunications systems have been utilized to send a wide variety of types of data: voice, video, written words, through facsimile, and computer communications. However, it has always been necessary to use separate lines for each type of communication. In the case of computer communications, a modem or conversion routine are also necessary for the transfer. ISDN is a limited set of standard interfaces to a digital communication network. The new ISDN standards utilize digital signals and will allow the transfer of more than one type of data along the same phone line. Modems will no longer be necessary and facsimile transmissions can be accomplished from the user's desk top computer. An ISDN network will provide end users with voice, data and certain image services on end to end digital circuits, using an international standard for interfaces that is accepted by communication carriers, users and systems manufacturers. The ISDN is based on the development of digital transmission and switching technologies and their use to construct an integrated digital network for telecommunication.
The switch-over from conventional telecommunications methods to ISDN has been hampered in part due to the need to develope entirely new equipment to handle the digital transmissions. This equipment will link all users into a large network via their telecommunications carrier. One essential piece of equipment to this network is a termination resistor primarily and secondarily termination for line fault protector to protect the integrity of the network if one network tap is experiencing electrical overvoltage/overcurrent conditions. Without such protection the entire network could be jeopardized. The present invention is such a device. Its importance cannot be overlooked.
In addition to providing this termination function; it would also be desirable for the package that includes the termination resistor to provide some protection from overcurrent/overvoltage fault conditions. This can be accomplished in a number of ways with varying degrees of effectiveness. For example, a fuse offers some protection, but too slow to be really effective over a reasonable range of fault conditions, and leaves the network unterminated if blown.
A capacitor offers some protection over a narrow range of fault conditions, but it is slow and can fail short, jeopardizing the network and user's equipment.
A transistor is effective over a narrow range of operating and fault conditions with unpredictable failure mode under fault conditions, i.e., could fail short or open.
A thermistor as used in the present invention is effective over a wide range of conditions providing quick, proportionally measured response to applied fault conditions, returning to normal operation when the fault condition is removed. Under extreme conditions such as a lightning bolt, the unit could fail open, and not short. Although electrical adapters which protect equipment from electrical surge conditions are known in the art, none of these previously developed adapters will function in an ISDN system. ISDN transmits a multitude of various types of data (voice, digital data) simultaneously, the electrical connection or line characteristics involved, primarily reflections must be eliminated so as to not affect the operation of the network. Previous terminator/resistor protection type devices used in pre-ISDN applications do not perform the same function as the present invention and do not provide the same protection to the network or user's peripherals. No known devices will handle the ISDN protocol with both surge protection and network termination.
For example, U.S. Pat. No. 4,799,901 to Pirc discloses a line fault protector for a conventional four wire phone line connection system for use with a modem. This device utilizes surface mounted bi-polar diodes which causes only clipping of the excess signal and not termination. Pirc cannot be adapted for use on an ISDN eight wire network and does not provide a termination function via a positive temperature coefficient thermistor that when used with an eight wire network then eliminates modems. U.S. Pat. No. 4,772,225 to Ulery discloses a technique for producing a 25 pin RS-232 type adaptor for placing small surface mounted devices within a network to provide line filtering. While this technique could be used to provide network termination or surge protection for conventional systems, it will not work with ISDN in its present form.
U S. Pat. No. 4,758,921 to Hung discloses a plug in surge protection unit using semi-conductors and fuses for use with quick clip terminal blocks on individual telephone subscriber's lines. This device does not provide line termination needed for use with ISDN network applications.
U.S. Pat. No. 4,742,541 to Cwirzen a telecommunications interface device for connecting conventional telephone lines to the customer's equipment via individual wires or a four (4) pin modular phone plug. The device does provide surge protection for communication systems, but its form and function are not adaptable to ISDN network applications with no termination features.
U.S. Pat. No. 4,729,752 to Dawson discloses a surge protection device which provides transient protection. The form and function of this device are not compatible with ISDN network applications.
U.S. Pat. No. 7,726,638 to Farrar discloses a device for providing surge protection, through the use of diodes, for existing circuits and for retrofitting existing connectors to provide surge protection. This device is primarily for internal computer use and does not provide the type of protection necessary for use with ISDN network applications.
U.S. Pat. No. 4,475,012 to Coulmance discloses a surge protection means within a conventional telephone set using positive temperature coefficient thermistor, transistors and diodes to protect and control the circuit. The disclosure has little applicability to ISDN type applications.
U.S. Pat No. 4,438,477 to Cawley disclosed a device to protect incoming and outgoing communication lines using positive temperature coefficient thermistors, metal oxide veristors, gas tubes and alternating current wall outlets which is inapplicable to ISDN systems or applications.
Other circuit interface and adaptor designs are disclosed is U.S. Pat. Nos. 4,438,477; 4,433,212; 4,313,147; 4,225,210;and, 3,750,082. None of these disclosures are relevant to ISDN systems.
It is the principal object of this invention to provide a network termination function coincident with line fault protection in a network.
Another object of the present invention to provide transient line fault protection against overcharge/overvoltage conditions caused by EMI, ESD or EMP for eight wire ISDN systems through the use of PTC thermistors and a thermoplastic housing.
Another object of this invention is to provide a resettable protective device which automatically returns to normal operation when the fault condition is removed.
Another object of the invention is to provide a means to test the integrity of the network by allowing the supplier to check each tap in the network, without disrupting service, to determine if termination is in fact occurring under fault conditions.
Numerous other advantages and features of the invention will become readily apparent from the detailed description of the preferred embodiment of the invention, from the claims, and from the accompanying drawings, in which like numerals are employed to designate like parts throughout the same.